1. Field of the Invention
The present invention relates to an architecture for a connection block in reconfigurable gate arrays and, in particular, to an architecture for reprogrammable interconnections of multi-context Programmable Gate Arrays (PGA) to implement connections between logic blocks and routing lines in reconfigurable gate arrays including connection blocks to connect inputs and outputs of different logic elements by means of connection wires.
2. Description of the Related Art
As is well known in this specific technical field, various architecture and circuit solutions have been proposed to implement the connection between routing lines and logic blocks in Programmable Gate Arrays.
To reduce the overload due to the processing time, multi-context reconfiguration architectures have been recently proposed to store some configurations in the array allowing context switchings in a very short time.
However this approach has some drawbacks: each SRAM cell used to store configuration bits must be repeated as many times as the contexts.
Most of the area occupation is due to the high number of SRAM memories allocated in the array, in particular those being used to determine which routing switchings must be activated to reach the desired connectivity between the logic blocks.
On the other hand the technology miniaturisation leads to programmable interconnections being responsible for most of the area occupation and of the delay. Therefore interconnections are an increasingly important key requirement for reprogrammable architectures, wherein devices like pass transistors, tristate buffers or multiplexers increase the area occupation and the capacitive load on wires and connectors, affecting the overall performance.
New solutions to optimise programmable interconnections are thus necessary to remove this difficulty.
FIG. 1 shows a typical programmable interconnection structure for reconfigurable gate arrays, in particular with reference to connection blocks, that implement the connection of inputs and outputs of different logic elements by means of connection wires.
This basic solution has been proposed by Kerry M. Pierce et al. in U.S. Pat. No. 5,760,604 granted on Jan. 3, 1996 (assigned to Xilinx, Inc.) and concerning an “Interconnect architecture for field programmable gate-array”.
This first solution provides the connection of each logic block input or output line by means of routing wires using switches formed by n-MOS pass transistors and a configuration cell memory enabling or disabling the connection. This choice requires a large silicon area, particularly in a multi-context structure.
The multiplexing diagram shown in FIG. 5a of U.S. Pat. No. 5,760,604 is the previously described switching structure. Only a switch is enabled for the connection of the output line corresponding to the logic block by means of an outer routing wire.
Some other circuit solutions have been used to connect wires using a CMOS transfer gate instead of a n-MOS pass transistor to preserve a high logic value of the signals, or arranging buffers before switches to improve the signal transmission rate.
However all these solutions require several configuration memory cells used to enable or disable switches.
FIG. 2 shows a sub-structure of a connection block. This traditional implementation refers to a line belonging to a connection block and it shows the high number or multi-context memories being required.
The area occupation highly depends on n, which is the vertical routing bus amplitude. However this solution is the ideal solution as for performance times since only one pass transistor passes through and in a connection block.
This second prior art solution has been described in U.S. Pat. No. 6,134,173 granted on Nov. 2, 1998 to R. G. Cliff, L. T. Cope, C. R. McClintock, W. Leong, J. A. Watson, J. Huang, B. Ahanin (assigned to Altera Corporation) and concerning a “Programmable logic array integrated circuit”.
Several detailed proposals to realize connection blocks have been disclosed by Richard G. Cliff et al, in the above-mentioned second patent.
While the connection block structure is quite similar to the above-described block (see FIG. 5 of the patent), an alternative diagram based on a multiplexing stage is also proposed. For example, the architecture shown in FIGS. 6–8 of U.S. Pat. No. 6,134,173 duplicates the input line, using a single memory cell to enable to switches located on two different wires. One of the two output wires can be selected by a multiplexer, thus programming a memory cell.
This diagram allows the number of programming cells to be reduced, but it increases the delay since the signal must pass through a switch and a multiplexer. A further alternative connection is proposed by using only a multiplexer (FIG. 7 of U.S. Pat. No. 6,134,173). In this case the connection between the logic block wire and the routing wire is fixed and a multiplexer connects the correct signal.
This structure minimizes the configuration memory cells but the signal has a delay depending on the multiplexer architecture and size. The last alternative architecture proposed by U.S. Pat. No. 6,134,173 is shown in FIG. 9 thereof wherein the output of a single multiplexer is connected to some logic blocks by means of a switch.
This diagram minimizes the number of multiplexers but it increases the signal delay, since it is necessary to pass through two stages.
Two similar multiplexer structures to be used for designing connection blocks are described in the following pages.
1) P. Chow, S. Seo, J. Rose, K Chung, G. Paez, I. Rahardja “The Design of a SRAM-Based Field Programmable Gate Array, Part 11: Circuit Design and Layout” in IEEE Transactions on Very Large Scale Integration (VLSI) Systems, Volume: 7, Published on: Sep. 3, 1999, pages: 321–330 and,
2) V. Baena-Lecuyer, M. A. Aguirre, A. Torralba, L. G. Franquelo, J. Faura “Decoder driven switching matrices in multicontext fpgas: area reduction and their effect on routability” In Proceedings of the 1999 IEEE International Symposium on Circuits and Systems. ISCAS'99, volume 1, pages 463–466, 1999
FIG. 3 of the first article and FIG. 2 of the second article show two multiplexer structures for minimizing the area occupation due to configuration memories.
These solutions simultaneously add a delay in the signal transmission and they do not require both control signal (true value and the opposite thereof) stored in each memory cell, the complete signal oscillation is limited by the transistor electric threshold on both high and low values.
On the other hand the first solution uses only n-MOS pass transistors (see FIG. 3), so that the complete signal oscillation is limited only on a high value, but it requires both control signals, provided by each memory cell to drive the switches accordingly. Therefore a level-shifter buffer is required at the end of the connection block in order to recover the electric threshold.